Packing material is widely used to prevent fluid leakage around an operating member in a housing with fluid, such as a rotary shaft or a sliding stem in fluid control valves or in a reciprocating pump shaft. Normally such packing is formed of a resilient member and is placed under a static load by being bolted into position within a packing box around the operating member. In other instances the packing is subjected to spring loading in what is known as a live loaded packing configuration. Live loaded packing is particularly useful in attempting to prevent leakage of undesired fluids into the environment. Also, it is desired to use commonly available packing material formed of polytetrafluorethylene (PTFE) because of the inertness of such material and its low frictional impact on an operating member.
However, it has been found that in attempts to use PTFE packing in a live loaded packing configuration where the PTFE packing is continually loaded to a stress level, i.e., 2,000-4,000 psi, (13790-27580 kPa) that the PTFE packing is caused to cold flow and extrude out of the sealing area. On the other hand, it is normally required to continually load the PTFE packing in the containment assembly in order to enable the packing to set in and maintain a tight seal against the operating member. Generally, it has been found necessary to continually load PTFE packing when operating at environmentally high pressures of about 1,000-2000 psi (6895-13790 kPa) which leads to undesired PTFE packing extrusion, fluid leakage, and increased wear and possible damage to the operating member.
Another problem which arises in the use of PTFE packing material is due to its thermal expansion characteristic which is approximately ten times that of metal. Thus, the volumetric expansion of PTFE packing must be considered in determining the amount of spring travel of the live loading configuration which is required to maintain an adequate load on the packing after it has been through a thermal cycle. This is a particular problem at high temperature operating conditions, i.e., at temperatures in excess of 250.degree. F. (121.degree. C.).
Prior attempts to solve the PTFE extrusion problem with anti-extrusion devices such as flat ring members on each side of the PTFE packing and having a tight fit with the operating member such as a valve stem, have not eliminated the problem where stringent emission control requirements must be met. If the ring-to-stem fit is made extremely tight to prevent extrusion, this causes difficulty in assembling the ring on the stem or shaft, an undue amount of stem friction, and can lead to a lockup of the valve stem or operating member. Reducing the tightness fit between the ring member and the valve stem eases assembly and reduces the friction but leads to PTFE extrusion in an amount which does not meet stringent fluid emission control requirements for certain fluids.
Accordingly, it is desired to provide a packing containment assembly with anti-extrusion provisions which prevents the extrusion of PTFE packing material and meets stringent fluid leakage control requirements while still enabling proper operation of the operating member being sealed by the packing material. In addition, it is desired to provide an improved packing arrangement enabling the amount of PTFE in the packing to be reduced which is particularly desirable when operating in environmental conditions of high temperature.